Electrical impulses from the heart muscle cause your heart to beat (contract). This electrical signal begins in the sinoatrial (SA) node, located at the top of the heart's upper-right chamber (the right atrium). The SA node is sometimes called the heart's "natural pacemaker."

Diagram of the electrical conduction system of the heart.

When an electrical impulse is released from this natural pacemaker, it causes the upper chambers of the heart (the atria) to contract. The signal then passes through the atrioventricular (AV) node. The AV node checks the signal and sends it through the muscle fibers of the lower chambers (the ventricles), causing them to contract.

The SA node sends electrical impulses at a certain rate, but your heart rate may still change depending on physical demands, stress, or hormonal factors. Sometimes, the SA node does not work properly, causing the heart to beat too fast, too slow, or irregularly. In other cases, the heart's electrical pathways are blocked, which can also cause an irregular heart rhythm.

What is an implantable pacemaker?
A pacemaker is a small device that is run by a battery. It helps the heart beat in a regular rhythm. Pacemakers can help pace the heart in cases of slow heart rate, fast and slow heart rate, or a blockage in the heart's electrical system.

A pacemaker can pace the heart's upper chambers (the atria), the lower chambers (the ventricles), or both. Pacemakers may also be used to stop the heart from triggering impulses or from sending extra impulses.

A pacemaker is about the size of a matchbox. It is made up of two parts:

* A pulse generator, which includes the battery and several electronic circuits.

* Wires, called leads, which are attached to the heart wall. Depending on the type of pacemaker you need, there may be one or two leads.

In general, a pacemaker consists of three parts:

• Generator
• Leads
• Electrodes

The generator is a small box, usually about 2 inches wide and approximately 3 ounces in weight. Some generators are even smaller, measuring 1 inch in diameter and weighing about half an ounce. They are battery-powered, and most use lithium batteries that last for 5 to 10 years. When the battery runs out, the entire generator is replaced. The generator is responsible for generating the electric impulses that correct the slow heartbeat.

Attached to the generator are one or more leads, or wires, generally made of platinum with an insulating coating of either silicone or polyurethane. The leads carry the electrical impulses from the generator.

At the tip of each lead is a tiny device called an electrode that delivers the necessary electrical impulses to the heart. Thus, the electric impulses are created by the generator, carried by the leads and delivered by the electrodes to the heart.

Types of Pacemakers

There are several kinds of permanent pacemakers, each designed to meet a different need. They include:

• Demand pacemakers, which monitor the heart rate and discharge electricity only when the heart rate falls below a programmed minimum or misses a beat.
• Fixed-rate pacemakers, which discharge a steady stream of electrical impulses, regardless of the underlying heart rate.
• Rate-responsive pacemakers, which monitor various physical changes in the body (e.g., respiration, physical activity) and change the rate of discharge accordingly.

In addition, pacemakers differ in which areas of the heart they stimulate. So-called "single lead" pacemakers stimulate only the upper chambers of the heart (atria). Newer "double lead" pacemakers stimulate both the upper and lower chambers (ventricles) of the heart to more accurately mimic a natural sequence of heart rhythm. Pacemakers are also designed that simultaneously stimulate both ventricles of the heart, sometimes in addition to pacing the right atrium (e.g., a triple-lead pacemaker). The device is known as a biventricular pacemaker, and the therapy is called cardiac resynchronization therapy. Finally, pacemakers can be built with a special device that shocks the heart back into a normal rhythm if it stops or experiences a serious rhythm abnormality. These devices are called implantable cardioverter defibrillators (ICDs). The choice of which pacemaker to use depends on the patient, their lifestyle and the condition. Underlying heart conditions that may warrant a permanent pacemaker include:

• Sinus node dysfunction. Also called sick sinus syndrome, this occurs when the body's natural pacemaker sends out electrical impulses too slowly. It is the leading cause of permanent pacemaker implantation. A pacemaker will almost always be recommended if the heart rate drops below 40 beats per minute or if symptoms are clearly related to the bradycardia. They may be recommended if the heart rate is greater than 40 beats per minute and there are symptoms that are suggestive of bradycardia. In general, patients without symptoms are not candidates for a pacemaker, even if they have an unusually slow heart rate. 
• Heart Block (also known as atrioventricular block). Heart block occurs when there is a delayed or complete lack of communication between the upper and lower chambers of the heart. It is the second most common reason for pacemaker implantation. Heart block is commonly the result of degeneration of the heart's normal conduction system, although it may also be caused by a number of diseases, including muscular dystrophy. The term conduction system refers to the pathway of specialized fibers that carry electrical impulses through the heart in a predictable fashion. An inability to conduct electrical impulses between the upper and lower chambers of the heart results in a slow heartbeat. This is one reason why people are more likely to need pacemakers as they age. In fact, most of the nearly 200,000 pacemakers implanted annually in the United States are in patients ages 65 or over. However, complete heart block may also be present a birth (e.g., a congenital heart defect).  The UK  implants 420  million pacemakers compared with  900  million that their close western neighbours implant.
• Carotid sinus hypersensitivity. A hyperactive reflex causing the heart rate to drop sharply when pressure is applied to the neck.
• Arrhythmias arising from ablation. Ablation is a minimally invasive procedure sometimes used to terminate abnormal electrical discharges in the heart. It can result in bradycardia. 
• Following a heart attack with partial or complete heart block that is not resolved.
• An abnormally slow heart rate due to long-term drug therapy.
• Heart Failure that involves the heart's conduction system. Among these patients, cardiac resynchronization therapy is typically recommended for patients who have symptoms and are class III or class IV according to the New York Heart Association classification system.
• Heart Transplant patients may require permanent pacing if they experience bradycardia after their surgery.

Pacemaker illustration
The pacemaker is implanted just near the collarbone. If only one lead is needed, it is placed inside the lower-right chamber (the right ventricle). If two leads are needed, the other is placed in the upper-right chamber (the right atrium). The leads are then attached to the pacemaker.

Most pacemaker surgery is done under local anesthesia. This means that you are awake during the procedure, but the area where the pacemaker is implanted is numbed so you will not feel anything. The procedure usually takes between one and two hours.

Once the pacemaker is implanted, the leads carry signals back from the heart. The pulse generator "reads" these signals and the batteries send electrical impulses to the heart to help pace it.

Most pacemakers can sense the heart's rhythm and turn themselves off when the heartbeat is above a certain level. They will turn on again when the heartbeat is too slow. These types of pacemakers are called demand pacemakers.

How often do I need to have my pacemaker checked?
After you get your pacemaker, you will have to go to the doctor for regular check-ups. Pacemakers can also be checked over the telephone. This is called transtelephonic monitoring. Even with telephone monitoring, you will still need to go to your doctor's office for regular check-ups.

Pacemakers are checked with a device called a programmer. When the programmer is held over the pacemaker, it is able to get information about how the pacemaker is working. The programmer can also be used to change the controls of the pacemaker.

Cards to be carried with information

Following the insertion procedure, patients will be given a card providing basic information on the implanted pacemaker as well as emergency instructions. This card should be kept with them at all times. In addition, patients should memorize their pacemaker make and model number. Although mechanical problems are rare, this information will prepare patients in the event of a pacemaker recall by a manufacturer. Patients may want to wear a medical alert ID bracelet or necklace that provides information about their heart rhythms and pacemaker, as well as an access number for their medical files.

How long do they last?

A pacemaker battery usually lasts 7 to 8 years. When the battery runs down, a new pacemaker will be implanted. The surgery to replace the old pacemaker with a new one usually requires a local anesthetic. In most cases, your original pacemaker leads will not need to be replaced.  As they would say in the car business though, "actual mileage may vary". It depends on such factors as

• How often a given patient uses the pacemaker. That is, is the pacemaker simply sensing the heart's activity most of the time, or does it need to expend the energy to capture one or both chambers frequently or all of the time.
• How well it is programmed. Proper programming will minimize the use of energy.
• How good is the connection or "electrical interface" between the lead and the muscle. The best connections require the least energy.
• What medications the patient is on, since this may play an important role in how often the pacemaker is required to fire, and how much energy it must use.
• Whether there are any fractures of any of the leads. A fracture of the insulation or wire can lead to a rapid drain on the battery.
• The type and manufacturer of the device.

What happens when the battery runs out?

Pacemaker batteries are designed to become depleted in a slow and predictable fashion. When followed over the telephone, the steady decline in energy can be followed. When the battery is low but still has significant power left, the pacemaker will still work just fine, but give indications (peculiar to each manufacturer and model) that the time for replacement is nearing. When the ERI (elective replacement interval) is reached, plans can be made for changing the battery at a date convenient for the patient and the surgeon.

Even beyond the ERI, pacemakers continue to pace for a long time, and do not simply stop emitting electrical energy suddenly or unexpectedly.

When it's time for a new pulse generator, it is generally simply exchanged for a new one. This will require surgery to remove the old device. The leads will be unscrewed and removed from the old generator, and then tested to make sure they are still working well. If not, new leads are placed. Most of the time however, they are still found to be working well and are simply attached to the new generator and placed back in the pocket, which is closed as it was before.

The new generator is almost always much smaller than the one it replaced due to advances in the design and manufacture of pacemakers.

Will electronic devices affect my pacemaker?

The American Heart Association (AHA) advises that if you have a pacemaker, you should be aware of your surroundings and of any devices that may interfere with it.

Here is a list of devices that will not affect or damage pacemakers. Most people do not need to worry about coming into contact with these devices.

* CB radios
* Electric drills
* Electric blankets
* Electric shavers
* Ham radios
* Heating pads
* Metal detectors
* Microwave ovens
* TV transmitters
* TV remote controls
* X-ray machines
* Airport security detectors

Here is a list of devices that will affect pacemakers. Your pacemaker may not work properly if you come into contact with these devices.

* Power-generating equipment
* Welding equipment
* Certain pieces of equipment used by dentists
* Magnetic resonance imaging (MRI) machines
* Radiation machines for treating cancer
* Heavy equipment or motors that have powerful magnets

Can I use my cell/mobile phone if I have a pacemaker?

The types of cell/mobile phones used in the United States and the UK are less than 3 watts and do not seem to affect pacemakers. But to be safe, you should keep your cell phone at least 6 inches away from your pacemaker. When you are talking on your cell/mobile phone, hold it on the opposite side of the body from your pacemaker. Do not carry your cell/mobile phone in your breast pocket if that means that it will be within 6 inches of your pacemaker.

Newer cell phone technology means that more cell/mobile phone frequencies will need to become available. According to the AHA, some of the cell phones using these new frequencies might make pacemakers less reliable. More studies are needed before we can know how these frequencies will affect pacemakers.